Since the reported induction of pluripotent stem cells (induced pluripotent stem (iPS) cells; also called as “artificial pluripotent stem cells” or “induced pluripotent stem cells”) from somatic cells (Non-patent Document 1), iPS cells have been produced by introducing reprograming factors into various mammalian cells including human and mouse cells (Non-patent Documents 1 to 9). Many of them use retrovirus vectors to introduce reprograming factors. However, since retrovirus vectors carry the risk of tumorigenesis as a result of integration into the host genome, their use is limited (Non-patent Document 3). To solve this problem, attempts of inducing iPS cells using adenovirus vectors or plasmids have been made, but as long as DNA-type vectors are used, it is impossible to completely remove concerns over integration into the genome. Furthermore, the induction efficiency of iPS cells by these vectors is extremely low (Non-patent Documents 10 to 13).
To solve these problems, the present inventors have previously developed a system for inducing iPS cells using a Sendai virus vector which is an RNA-type virus (Patent Document 1). Induction efficiency of iPS cells using the Sendai virus vector was significantly higher than that in previous cases using other vectors. Since the Sendai virus vectors do not have a DNA phase during their lifecycle, there is no concern that they will become integrated into the host genome, and they are excellent in terms of safety. Also, the vectors can be easily removed after induction of iPS cells. However, techniques that use Sendai virus vectors for further increasing the induction efficiency of iPS cells are not known.